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Ch. Rajesh Int. Journal of Engineering Research and Applications www.ijera.com ISSN : 2248-9622, Vol. 4, Issue 6( Version 6), June 2014, pp.58-64 RESEARCH ARTICLE OPEN ACCESS Performance Analysis of STATCOM under Various Line Faults Ch. Rajesh*, G.Basava Sankara Rao** *(Department of Electrical And Electronics Engineering, R.V.R & j.C College of Engineering, Guntur (A.P), INDIA) **(Department of Electrical And Electronics Engineering, R.V.R & j.C College of Engineering, Guntur (A.P), INDIA) ABSTRACT Reactive power control using the static compensator (STATCOM) has more advantageous due to outstanding performance of STATCOM. In transmission and distribution systems, the reactive power control is done by using the STATCOM based on voltage source converter (VSC).STATCOM can supply large amount of VAR’s during system faults for voltage support. The STATCOM effects the VSC over currents and trips, during the power system faults when VAR’s support is more required. In this paper, we propose and develop an “emergency PWM” strategy to prevent over-currents (and trips) in the VSC during and after single line to ground system faults, LLLG faults and to ensure that the STATCOM supplies required reactive power. System performance during a nonlinear load connected without any fault is also considered. The Simulation results are shown for a 48-pulse VSC based ± 100 MVAR STATCOM connected to a 2- bus power strategy to prevent VSC over-currents and to supply required reactive power under line to ground system faults. Keywords –Voltage Source Converter (VSC), STATCOM, Emergency Pulse-width Modulation (PWM), Single line to ground fault. I. INTRODUCTION Flexible AC Transmission systems(FACTS) controllers are emerging as an effective and promising alternative to enhance the power transfer capability and stability of the network by redistributing the line flow and regulating the bus voltages. Static VAR compensator (SVC) and Thyristor controlled series compensator (TCSC) are some of the commonly used FACTS controllers , The developments in the field of power electronics, particularly Gate Turn-off (GTO) based devices, have introduced a new family of versatile FACTS controllers, namely static synchronous compensator (STATCOM) ,The STATCOM is one of the custom power devices that received much attention for improving system stability, with the development of power electronics technology, custom power devices play important role in bringing unprecedented efficiency improvement and cost effectiveness in modern electrical power system [1,2]. The custom power is relatively new concept aimed at achieving high power quality, operational flexibility and controllability of electrical power systems [3-5]. The possibility of generating or absorbing controllable reactive power with various power electronic switching converters has long been recognized [6-8]. The STATCOM based on voltage source converter (VSC) is used for voltage regulation in transmission and distribution systems[8-10]. The STATCOM can rapidly supply dynamic VAR’s during system faults for voltage support. In this paper, we propose and develop an “emergency PWM” strategy to prevent www.ijera.com over-currents (and trips) in the VSC during line to ground faults, all though PWM technique results in higher switching losses but it recompense total system loss. This limitation of implementing VSC with PWM functionality, results in avoiding overcurrents and trips of the STATCOM supplies required reactive power. With “emergency PWM” strategy STATCOM gains capability to prevent overcurrents and trips in the VSC based STATCOM. Simulation results are presented for a 48-pulse VSC based ±100 MVAR STATCOM connected to a 2-bus power system. The operating characteristic of compensator during steady state, capacitive and inductive modes validate “emergency PWM” strategy [13] to prevent VSC over-currents and to supply required reactive power under line to ground system faults[9-12]. II. BASIC STRUCTURE OF VOLTAGE SOURCE CONVERTER(VSC) Fig. 1 shows the 48-pulse voltage source converter topology for ST ATCOM application. The VSC consists of four (lnv 1 - Inv4) 3-level Neutral Point Clamped (NPC) converters which are connected in series by four (Tl-T4) transformer coupling. The primary side of the transformer is connected in series as shown in Fig. 1. Due to the strict loss outlay for STATCOM application, each VSC is operated at fundamental frequency switching or in square-wave mode. The gating of VSCs is Phase-shifted so as to yield 48 pulse output voltage waveform with series transformer coupling on the 58 | P a g e Ch. Rajesh Int. Journal of Engineering Research and Applications www.ijera.com ISSN : 2248-9622, Vol. 4, Issue 6( Version 6), June 2014, pp.58-64 primary side. The performance of the STATCOM under system faults (such as single line-ground faults) results in converter over currents and STATCOM trips. Figure. l The 48 –pulse voltage source converter circuit for ±100 MVA STATCOM application Fig.2 shows the 2-bus 500 kV power system simulation model with 48 pulses implemented VSC based ±100 MVAR STATCOM. Fig.2 shows the implemented angle controlled (α) STATCOM controller. An inner feedback loop is used to regulate the STATCOM instantaneous reactive power current Iq shunt, reminding that this control is achieved only by controlling α, of the inverter output voltage relative towards the transmission line voltage, this technique makes it possible to maintain a constant maximum ratio between the inverter Output voltage and the VSC dc-capacitor. The reference value for the reactive current control loop is generated by an outer loop responsible for the system voltage control (Vbus_ref). This outer control loop is similar to that used in conformist static VAR compensators, and includes an adjustable slope/droop setting that defines the voltage error at full STATCON reactive output. There is an unavoidable delay in the feedback of the voltage regulating loop because of the time taken to compute the positive sequence fundamental bus voltage (Vbus). as a result an extremely fast response (typically 1⁄4 cycle) can be achieved for the reactive current controller (Iq. Shunt), the response time of the voltage regulator is typically about half Cycle of the line voltage. The 48-pulse converter is comprised by four 12pulse converter linked by four 12-pulse transformers with phase-shift windings. The 48-pulse converter can be used in high power applications without AC filters due to its high performance and low harmonic rate on the AC side. The output voltages have harmonics n = 48r ± 1, where r = 0, 1, 2... i.e., 47th, 49th, 95th, 97th... with magnitudes of 1/47th, 1/49th, 1/95th, 1/97th... respectively, respect to the www.ijera.com fundamental; on the DC side the lower circulating harmonic current will be the 48th. The phase-shift pattern on each 12-pulse converter is the following: 1th 12-pulse converter PST: +7.50 to eliminate the 24-pulse harmonics +3.750 to eliminate the 48-pulse harmonics Total +11.250 Winding turn rate 1:tan (11.250) Driver: -7.50 to eliminate the 24-pulse harmonics -3.750 to eliminate the 48-pulse harmonics Total -11.250 2nd 12-pulse converter PST: -7.50 to eliminate the 24-pulse harmonics +3.750 to eliminate the 48-pulse harmonics Total -3.750 Winding turn rate 1:tan (3..750) Driver: +7.50 to eliminate the 24-pulse harmonics -3.750 to eliminate the 48-pulse harmonics Total +3.750 3th 12-pulse converter PST: +7.50 to eliminate the 24-pulse harmonics -3.750 to eliminate the 48-pulse harmonics Total +3.750 Winding turn rate 1:tan (3..750) Driver: -7.50 to eliminate the 24-pulse harmonics +3.750to eliminate the 48-pulse harmonics Total -3.750 4th 12-pulse converter PST: -7.50 to eliminate the 24-pulse harmonics -3.750 to eliminate the 48-pulse harmonics Total -11.250 Winding turn rate 1:tan (3..750) Driver: +7.50 to eliminate the 24-pulse harmonics +3.750 to eliminate the 48-pulse harmonics Total +11.250 III. CONTROL STRATEGY The proposed solution is based on "emergency PWM" mode, where the VSCs will individually detect and self implement PWM switching to control their phase (VSC pole and device) currents within predetermined limits. Each VSC will ensure that its over-current limit is not reached during and after a system fault, and under any bus voltage condition (including negative sequence and harmonics). This control strategy enables the STATCOM to remain online and recovering from a system fault, when its V AR support is required the most. Fig.6 and Fig.7 shows the VSC phase voltages and currents under normal and faulted conditions with "emergency pwm". The phase current rapidly increases at the onset of the fault and is typically higher than the over-current limit of the VSC devices. This "emergency PWM" concept is illustrated in such a way that the VSC phase voltage is modulated to control the phase (VSC pole and device) current during the fault. It is seen that the VSC phase current is controlled such that the STATCOM still delivers required reactive power (or current) during the fault. The extra switching’s in the VSC will result in higher losses during this period. However, the priority is to 59 | P a g e Ch. Rajesh Int. Journal of Engineering Research and Applications www.ijera.com ISSN : 2248-9622, Vol. 4, Issue 6( Version 6), June 2014, pp.58-64 keep the STATCOM online to support the bus voltage during and recovering from system faults. Figure 4: α-β coordinates transformation Figure.2 Single Line Diagram of Simulation Model The p-q theory performs a Clarke transformation of a stationary system of coordinates a,b,c to an orthogonal reference system of coordinates α,β. In a,b,c coordinates axes are fixed on the same plane, apart from each other by 120o that as shown in Fig 4. The instantaneous space vectors voltage and current Ea , Ia are set on the a-axis, Eb , Ib are on the b axis, and Ec , Ic are on the c axis. These space vectors are easily transformed into α,β coordinates. The instantaneous source voltages Eas, Ebs, Ecs are transformed into the α,β coordinate’s voltage 𝐸𝛼 , 𝐸𝛽 by Clarke transformation as follows: 𝐸𝛼 𝐸𝛽 = 2 3 1 0 −1 −1 2 3 2 − 3 2 2 𝐸𝑎𝑠 𝐸𝑏𝑠 𝐸𝑐𝑠 (1) Similarly, the instantaneous source current Ias, Ibs, Ics also transformed into the α,β coordinate’s current 𝐼𝛼 , 𝐼𝛽 by Clarke transformation that is given as; −1 −1 𝐼𝑎𝑠 1 𝐼𝛼 2 2 2 𝐼𝑏𝑠 = (2) 3 − 3 𝐼𝛽 3 0 𝐼 𝑐𝑠 2 2 Where α and β-axes are the orthogonal coordinates. The 𝐸𝛼 , 𝐼𝛼 are on the α-axis, and 𝐸𝛽 , 𝐼𝛽 are on the β-axis. Figure. 3 STATCOM control block diagram IV. CONTROL METHOD:INSTANTANEOUS REAL POWER THEORY The proposed instantaneous real-power (p) theory derived from the conventional p-q theory or instantaneous power theory concept and uses simple algebraic calculations. It operates in steady-state or transient as well as for generic voltage and current power systems that allowing to control the active power filters in real-time. The active filter should supply the oscillating portion of the instantaneous active current of the load and hence makes source current sinusoidal. www.ijera.com Real-Power (p) calculation: The orthogonal coordinates of voltage and current 𝐸𝛼 , 𝐼𝛼 are on the α-axis, and 𝐸𝛽 , 𝐼𝛽 are on the β-axis. Let the instantaneous real-power calculated from the α-axis and β- axis of the current and voltage respectively. These are given by the conventional definition of real-power as : 𝑃𝑎𝑐 = 𝐸𝛼 𝐼𝛼 + 𝐸𝛽 𝐼𝛽 (3) This instantaneous real-power Pac is passed to first order Butterworth design based 50 Hz low pass filter (LPF) for eliminating the higher order components; it allows the fundamental component only. These LPF indicates ac components of the realpower losses and it’s denoted as Pac(loss). 60 | P a g e Ch. Rajesh Int. Journal of Engineering Research and Applications www.ijera.com ISSN : 2248-9622, Vol. 4, Issue 6( Version 6), June 2014, pp.58-64 The DC power loss is calculated from the comparison of the dc-bus capacitor voltage of the cascaded inverter and desired reference voltage. The proportional and integral gains (PI Controller) are determining the dynamic response and settling time of the dc-bus capacitor voltage. The DC component power losses can be written as 𝑃𝑑𝑐 (𝑙𝑜𝑠𝑠 ) = 𝐸𝑑𝑐 𝑟𝑒𝑓 − 𝐸𝑑𝑐 [𝑘𝑝 + 𝑘1 𝑠 𝑃 } 0 (6) From this equation, we can calculate the orthogonal coordinate’s active-power current. The -axis of the instantaneous active current is written as: I𝑝𝛼 = 𝐸 𝛼 𝐸𝛼 𝑃 2 +𝐸 2 𝛽 (7) Similarly, the -axis of the instantaneous active current is written as: I𝑝𝛽 = 𝐸 𝛼 𝐸𝛽 𝑃 2 +𝐸 2 𝛽 + 𝐸𝛽 𝑡 𝐸𝛽 𝑃 𝐸𝛼 2 +𝐸𝛽 2 (10) The AC and DC component of the instantaneous power p(t) is related to the harmonics currents. The instantaneous real power generates the reference currents required to compensate the distorted line current harmonics and reactive power. V. SIMULATION AND RESULTS The instantaneous current on the coordinates of Ipand Ipare divided into two kinds of instantaneous current components; first is real-power losses and second is reactive power losses, but this proposed controller computes only the real-power losses. So the coordinate currents Ic,Icare calculated from the E,Evoltages with instantaneous real power P only and the reactive power Q is assumed to be zero. This approach reduces the calculations and shows better performance than the conventional methods. The coordinate currents can be calculated as 𝐸𝛽 −𝐸𝛼 𝐸𝛼 𝑃 𝐸𝛼 2 +𝐸𝛽 2 (4) The instantaneous real-power ( P) is calculated from the AC component of the real-power loss Pac(loss) and the DC power loss Pdc(Loss) ; it can be defined as follows; 𝑃 = 𝑃𝑎𝑐 (𝑙𝑜𝑠𝑠 ) + 𝑃𝑑𝑐 (𝑙𝑜𝑠𝑠 ) (5) 𝐼𝑝𝛼 𝐸𝛼 1 = 2 2 { (𝐸𝛼 +𝐸𝛽 ) 𝐸𝛽 𝐼𝑝𝛽 𝑃 𝑡 = 𝐸𝛼 𝑡 The system simulation diagram is shown in Figure 4 with a 2-bus 500 kV power system. The ±100MV AR STATCOM is implemented with a 48pulse VSC and is connected to a 500 kV bus as shown in Figure 2. A general fault generator is implemented at bus 2, which results in a voltage dip at the STATCOM bus. Attention is focused on single line-ground faults and STATCOM performance with the proposed "emergency PWM' concept in this section. Results given in per unit values, with 1.0 P.U as 500 kV. During steady state operation VSC voltage is in phase with system voltage. If the voltage generated by the VSC is higher (or lower) than the system voltage, then STATCOM generates(or absorbs) reactive power. The amount of reactive power depends on the VSC voltage magnitude and on the transformer leakage reactances. The fundamental component of VSC voltage is controlled by varying dc bus voltage. In order to vary dc voltage and therefore the reactive power, the VSC voltages angle (alpha) which is normally kept at close to zero is now phase shifted. This VSC voltage may lag or lead and produces a temporary flow of active power which results in increase or decrease of dc capacitor voltages. With help of emergency pwm the output voltage distortion and capacitor ripple current can be reduced to any desired degree. Thus static VAR generator, employing a perfect voltage sourced converter, would produce sinusoidal output voltages, would draw sinusoidal reactive current from ac system. (8) Let the instantaneous powers p(t) in the -axis and the - axis is represented as pand prespectively. They are given by the definition of real-power as follows: 𝑃 𝑡 = 𝐸𝑝𝛼 𝑡 𝐼𝑝𝛼 𝑡 + 𝐸𝑝𝛽 𝑡 𝐼𝑝𝛽 𝑡 (9) Figure 4 VSC voltage and current waveforms under normal condition From this equation (9), substitute the orthogonal coordinates -axis active power (7) and -axis active power (8); we can calculate the real-power P(t) as follows www.ijera.com 61 | P a g e Ch. Rajesh Int. Journal of Engineering Research and Applications www.ijera.com ISSN : 2248-9622, Vol. 4, Issue 6( Version 6), June 2014, pp.58-64 Fig.5 Simulink circuit of STATCOM connected system Fig.5 shows STATCOM operation in voltage regulation mode with emergency PWM under fault conditions. During fault conditions the inverter currents are very high this is the main reason for tripping and by implementing VSC with PWM functionality, avoids over-current and trips and in Fig.6 It is clearly shown that bus voltage, injected currents are optimum, this ensures that STATCOM is in online and function without tripping, Fig. 7 shows reactive power and active supplied by the STATCOM under critical conditions. it is varying from inductive to capacitive within 0.3 to 0.7sec which shows STATCOM supplying adequate reactive power under fault condition. Fig.8 shows the STATCOM controller voltages and currents which are in permissible limits under fault conditions. STATCOM operates in two modes either capacitive are inductive mode, these are known according to Var variations. And dc link voltage always resembles STATCOM response, it is constant under normal stipulation. When ac voltage reduces STATCOM reacts fastIy and supplies necessary reactive power. At this condition reactive power Q is positive which resembles it is in capacitive mode, where as operation is vice versa when ac voltage increases. Figure.6 VSC voltage and current under LG fault www.ijera.com Figure.7 STATCOM reactive power Q in MVAR and active power P in MVA Figure.8 DC link-voltge LLL-G FAULT LLLG faults are very rarely occur in the power systems but the effect os this fault is very severe. Fig.9 shows dynamic response of STATCOM under LLL-G faults, as these faults are sever, at this condition the inverters currents are very high than rated, but still STATCOM continuous to be in online without tripping and after a particular interval of time system comes to steady state. This is verified through Fig 11(a). The moment fault occurred, bus voltages starts to fall but STATCOM responds abruptly and commence within minimum interval of time and starts supplying reactive power, these can be seen in Fig 10. From Fig.11, we can examine dc link voltage variations, under fault conditions and once STATCOM starts supplying then it gradually settles down i.e at point 0.3 sec. 62 | P a g e Ch. Rajesh Int. Journal of Engineering Research and Applications www.ijera.com ISSN : 2248-9622, Vol. 4, Issue 6( Version 6), June 2014, pp.58-64 VI. Figure. 9 VSC voltage and current under LLLG fault CONCLUSION This paper describes dynamic performance of STATCOM when it is effected to L-G and LLL-G faults. The operating characteristic of STATCOM during steady state, capacitive and inductive modes of operation has been reasonably acceptable and competitive for design of an economical dynamic static compensator and by implementing "emergency PWM" strategy STATCOM gains capability to prevent over-currents and trips in the VSC based STATCOM. Simulation results are presented for a 48-pulse VSC based ±100 MVAR STATCOM connected to a 2-bus power system. Bus voltages, and primary injected currents of STATCOM, under normal and faulted conditions shown in detail. In addition to this a nonlinear load is connected and operated at no fault conditions, and harmonics are eliminated in the source current. This enables online operation of the STATCOM and supplies required reactive power when it is most required. Thus the performance of STATCOM has improved with the new control strategy. REFERENCES [1]. [2]. [3]. Fig.10 STATCOM reactive power Q in MVAR and active power P in MVA [4]. [5]. [6]. [7]. [8]. Figure.11 DC link-voltage www.ijera.com N.G. Hingorani, "Power Electronics in Electric Utilities: Role Of Power Electronics In Future Power Systems," Proceedings of the IEEE, vol. 76, pp. 481, 1988. N.G. Hingorani and L.Gyugyi, "Understanding FACTS: concepts And Technology of Flexible AC Transmission Systems: IEEE press, 2000. C. Schauder et aI., "Development of A ±100 Mvar Static Condenser For Voltage Control Of Transmission Systems," IEEE, PES Summer Power Meeting, Paper No. 94 sm 479-6 pwrd, 1994. C. Schauder et al. "TVA STATCON project: design, installation and Commissioning," CIGRE paper 14-106, 1996 N.G. Hingorani et al. "Static Condenser – Prototype Application," CIGRE Paper, New Zealand, 1993. S.BhattachaIya, Z. Xi, " A Practical Operation Strategy for STATCOM Under Single Line To Ground Faults In Power System" ,IEEE PSCE conf.,Nov.2006,Atlanta. L.Gyugyi, "Dynamic Compensation of Ac Transmission Lines by Solid-State Synchronous Voltage Sources," IEEE, PES Summer Power Meeting, Paper No. 93 sm 434-1 pwrd, 1993 L.Gyugyi, "Reactive Power Generation And Control By Thyristor Circuits," IEEE Trans. Ind. Appl., vol. la-15, no. 5, pp. 521-532, Sept.ioct., 1979 63 | P a g e Ch. Rajesh Int. Journal of Engineering Research and Applications www.ijera.com ISSN : 2248-9622, Vol. 4, Issue 6( Version 6), June 2014, pp.58-64 [9]. [10]. [11]. [12]. [13]. J.B.Ekanayake and M.Jenkins, "A ThreeLevel Advanced Static Var Compensator," Power Delivery, IEEE Transactions on, vol. 11, pp. 540, 1996. C.W. Edwards et aI., "Advanced Static Var Generator Employing GTO Thyristors," IEEE, PES winter Power Meeting, Paper No. 38wml09-1, 1988. Q.J.Liu,Y.Z.Sun, T.L.Shen“Adaptive nonlinear coordinated excitation and STATCOM Controller based on Hamiltonian structure for multimachine power System stability enhancement.” IEEE Proceedings on control Theory Appl. Vol. 150, No 3, pp: 285-294, May 2003. Amit K Jain, AmanBehal“Non Linear Controllers for fast voltage regulation using STATCOM.” IEEE transactions on control system technology, Vol. 12, No 6, pp: 827842, Nov 2004. Amir H. Norouzi, A.M.Sharaf“Two control schemes to entrance the dynamic performance of the STATCOM & SSSC.”, IEEE Transactions on Power delivery, Vol. 20, pp: 435-422, Jan 2005. Ch.Rajesh received B.Tech degree from Prasad V Potluri Siddhartha Institute of Technology in the year 2012.Presently, He is pursuing his M.Tech in Power Systems from RVR&JC College of Engineering. His major interests are power quality improvements in power systems and voltage mitigation. G.Basava Sankara Rao received his U.G from A.M.I.E. He received his M.Tech in High Voltage Engineering from JNTU KAKINADA. Currently, He is working as a faculty in RVR&JC College of Engineering, Guntur. He has 29years of teaching experience. His major interests are researches related to high voltage engineering and design of relaying equipment for various applications. www.ijera.com 64 | P a g e